1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser element wherein a semiconductor laminated structure having an active layer is etched to form a mesa, and the circumference of the mesa is buried with a burying layer.
2. Background Art
In a semiconductor laser element, the current pathway must be narrowed to efficiently supply a current to the active layer. Therefore, in many semiconductor lasers, after fabricating a semiconductor laminated structure having an active layer, a mesa is formed using a technique to transfer micro-patterns to an inductive film and an etching technique to limit the region where a current flows, and to narrow the current pathway. At this time, from the standpoint of the protection of the active layer exposed on the side of the mesa, heat dissipation, or the parasitic capacity of the element, a buried structure wherein the circumference of the mesa is coated with semiconductors is formed.
FIG. 11 is a sectional view showing a semiconductor laser element wherein the circumference of the mesa in a semiconductor laminated structure having an active layer is buried with n/p/n/p-type semiconductor layers. A mesa of a semiconductor laminated structure wherein a p-type InP clad layer 12, an AlGaInAs lower optical confinement layer 13, an AlGaInAs-MQW active layer 14, an n-type AlGaInAs upper optical confinement layer 15, and an n-type InP clad layer 16 are sequentially grown on a p-type InP substrate 11, is formed. The circumference of the mesa is buried with p-type InP burying layers 17a and 17b, an n-type InP current blocking layer 18, a p-type InP burying layer 19, and an n-type InP burying layer 20. Thereon, an n-type InP contact layer 21, an n-type InGaAs contact layer 22, and an n-type InP cap layer 23 are formed.
Here, the side of the mesa must be coated with the p-type InP burying layer 17a. This is because if the n-type InP current blocking layer 18 contacts the mesa, a current flows from the mesa to the burying layers, and the current to the active layer 14 cannot be narrowed.
The growth of the p-type InP burying layer 17a on the side of the mesa is much influenced by the surface state and the shape of the side of the mesa, which becomes the burying boundary. The portion exposed on the surface may be oxidized depending on the methods or conditions of etching for forming the mesa, or the semiconductor materials composing the mesa. On such an oxide layer, since the p-type InP burying layer 17a is difficult to form epitaxially, growing so as to coat such a portion begins after the semiconductor layers below the surface-oxidized semiconductor layer has been completely buried.
Therefore, there has been a problem wherein the n-type InP current blocking layer 18 contacts the mesa, and an invalid current pathway 24 wherein a current flows from the mesa to the burying layers is formed as shown in FIG. 12. Also in the mesa composed of Al-containing semiconductor materials, Al on the side of the mesa is oxidized when exposed to the atmosphere disturbing burying growth, and defective burying such as pitted growth occurs easily. In addition, since oxygen or impurities on the boundary might cause crystalline defects, there has been a problem wherein they become the cause of element deterioration. For these reasons, it is important for the characteristics and reliability of semiconductor lasers to reduce the amount of oxygen on the side of the mesa composed of Al-containing semiconductor materials.
Conventionally, therefore, the oxidation of Al-containing semiconductor layers has been prevented by placing a wafer in the state having a protective layer on the Al-containing semiconductor layer in a burying growth furnace, and, after surface cleaning is performed using HCl gas at a low temperature of not higher than 450° C. to remove oxygen on the surface of the protective layer, elevating the temperature to 450° C. or higher to perform mesa formation and burying growth using HCl gas etching (e.g., refer to Japanese Patent Laid-Open No. 7-263355). However, if the Al-containing semiconductor layer is exposed to the atmosphere and oxidized, it is difficult to remove oxygen only by HCl cleaning at a temperature of 450° C. or lower.